1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an in-plane switching (IPS) mode liquid crystal display device.
2. Description of the Related Art
Liquid crystal display (LCD) devices include an upper substrate, a lower substrate, and a liquid crystal layer interposed between the substrates. The LCD devices further include polarizers and retardation films at outer surfaces of the upper and lower substrates. The polarizers and retardation films are selectively used to change polarization, transmittance and/or refraction direction of a light. Thus, LCD devices having high brightness and contrast may be achieved. The LCD devices may be widely used for notebook computers because of a slim structure and low power consumption.
A twisted nematic (“TN”) mode is one type of the LCD devices. The TN mode LCD devices are frequently used but they may not be suitable for a large size display device because a light transmittance in a gray scale varies according to viewing angles. Further, in the TN mode LCD devices, the light transmittance is almost symmetric over a broad range of viewing angles from left and right directions with respect to a center portion of the devices. On the other hand, the light transmittance may not be symmetric at viewing angles from upper and lower directions with respect to the center portion of the devices. Therefore, images may be reversed in a certain range at viewing angles from the upper and lower directions, and viewing angles of the device may be narrowed.
In-plane switching mode liquid crystal display (“IPS-LCD”) devices may provide an improved viewing angle. In the IPS-LCD devices, electrodes for driving liquid crystal molecules are formed on the same substrate, and the liquid crystal molecules are driven by an electric field parallel to the substrates. The IPS-LCD devices have improved viewing angle characteristics such as contrast, gray inversion and color shift.
FIG. 1 is a schematic plan view illustrating a pixel of an IPS LCD device 10 according to the related art. In FIG. 1, a gate line 13 and a data line 15 are formed on a substrate and cross each other to define a pixel region P. A pixel electrode 17 and a common electrode 19 are formed in the pixel region P. The pixel electrode 17 and the common electrode 19 are disposed in an alternate pattern as shown in FIG. 1. Although not shown, a thin film transistor is formed at a crossing portion of the gate line 13 and the data line 15. Liquid crystal molecules 21 are driven by an electric field 23 induced between the pixel electrode 17 and the common electrode 19 when voltage is applied to the pixel electrode 17 and the common electrode 19. The electric field 23 is parallel to the substrate.
In the IPS LCD device, the liquid crystal molecules 21 are arranged along the same direction due to the electric field 23 between the pixel electrode 17 and the common electrode 19. The arranged liquid crystal molecules 21 form a domain and multi-domains are formed in one pixel region. As a result, the IPS LCD device may have a wide viewing angle.
FIG. 2 is a schematic cross-sectional view illustrating an IPS LCD device of FIG. 1. In FIG. 2, an upper substrate 29 and a lower substrate 33 are attached with the liquid crystal molecules 21 disposed therebetween. The upper substrate 29 includes a color filter layer (not shown) on an inner surface thereof, and the lower substrate 33 includes pixel and common electrodes (not shown) on an inner surface thereof. A rear electrode 27 is formed on an outer surface of the upper substrate 29, and an upper polarizer 31 is disposed over the rear electrode 27. The rear electrode 27 is formed of a transparent conductive material such as indium tin oxide. The rear electrode 27 cuts off static electricity from the outside and prevents deterioration of image quality.
A lower polarizer 35 is disposed over an outer surface of the lower substrate 33, and a backlight unit 37 is disposed over the lower polarizer 35. The backlight unit 37 includes several optical films such as a diffusion sheet and a prism sheet so that light is uniformly provided all over the surface of the lower substrate 33.
FIG. 3 is a schematic plan view illustrating the IPS LCD device of FIG. 2. In the IPS LCD device, a common voltage, which is also provided to a gate driving unit, is applied to a lower substrate including thin film transistors through a tape carrier package (TCP). In FIG. 3, upper and lower substrates 29 and 33 are attached such that peripheral portions at adjacent two sides of the lower substrate 33 are exposed by the upper substrate 29. A data driving unit 40 and a gate driving unit 42 are connected to the peripheral portions of the lower substrate 33 through tape carrier packages (TCPs) 44 and provide signals to elements of the lower substrate 33. The data driving unit 40 and the gate driving unit 42 may be formed on printed circuit boards (PCBs). The TCPs 44 are flexible and include driver integrated circuits (ICs) 50. The driver ICs 50 output image signals and/or scan signals. In addition, dummy lines may be formed in the peripheral portion of the lower substrate 33.
The data and gate driving units 40 and 42 are folded toward and placed on an outer surface of a top case (not shown) when the top case is combined with the attached substrates 29 and 33.
The data driving unit 40 includes a common voltage generating circuit (not shown). A common voltage Vcom is generated from the common voltage generating circuit and is output to a common voltage output node 52. The common voltage Vcom is applied to the common electrode 19 of FIG. 1 on the lower substrate 33 through the TCP 44. When the top case is combined with the substrates 29 and 33 and the data and gate driving units 40 and 42 are folded toward the outer surface of the top case, the common voltage output node 52 is connected to a common voltage input node 54 of the gate driving unit 42. The common voltage Vcom is also applied to the common electrode 19 of FIG. 1 on the lower substrate 33 through the gate driving unit 42 and the TCP 44.
In the IPS LCD device 10, the TCPs may be disconnected or mismatched with the common electrode during the manufacturing process. Furthermore, contact resistance exists between the TCPs and the lower substrate. Therefore, when the common voltage is applied to the common electrode, signals may be distorted. The distortion of the signals may exacerbate due to parasitic capacitances in pixels on the lower substrate, and a greenish phenomenon or shutdown cross-talk may occur because of imperfect response of the liquid crystal molecules.